arXiv subjects

Joël Leymarie

Publications and source records attributed to Joël Leymarie.

4 recordsLinked to original sources

arXiv2025

Time-resolved dynamics of GaN waveguide polaritons

We implement a new experimental approach to directly measure the lifetime of guided polaritons arising from the strong-coupling of GaN excitons and the guided photonic modes of a slab waveguide. Using a Fourier imaging setup, combined with spatial filtering of the emission, the emission associated to polaritonic modes with well-defined propagation constants can be selectively analyzed in the temporal domain. By directing it to the entrance slit of a streak camera, time-resolved photoluminescence (TRPL) measurements along the polariton dispersion branch were performed at 40 K, enabling to assess the time decay of polariton modes. By combining this information with the photonic/excitonic fraction corresponding to each polariton mode, extracted from a coupled-oscillators model that indicate a Rabi splitting of $Ω$ = 80 meV, we could extract the photon lifetime in the waveguide $τ_γ\, =\, 3\pm 1$ ps. This corresponds to a record $Q$-factor in the UV of 16 000. The excitonic reservoir lifetime, which contributes to polariton formation, was determined through TRPL measurements on excitonic luminescence. Finally, measurements conducted at lower temperature highlight secondary feeding mechanisms for the guided polaritonic mode, either via photon recycling from the AlGaN cladding layer or through resonant injection of photons from transitions below the band gap.

cond-mat.mtrl-sci↗

arXiv2023

Experimental demonstration of a Two-Dimensional Hole Gas (2DHG) in a GaN/AlGaN/GaN based heterostructure by optical spectroscopy

The polarization discontinuity across interfaces in polar nitride-based heterostructures can lead to the formation of two-dimensional electron and hole gases. In the past, the observation of these electron and hole gases has been achieved through various experimental techniques, most often by electronic measurements but occasionally by optical means. However, the occurrence of a two-dimensional hole gas has never been demonstrated optically. The objective of this article is to demonstrate, thanks to the combination of various optical spectroscopy techniques coupled to numerical simulations, the presence of a two-dimensional hole gas in a GaN/AlGaN/GaN heterostructure. This is made possible thanks to a GaN/AlGaN/GaN heterostructure displaying a micrometer-thick AlGaN layer and a GaN cap thicker than in conventional GaN-based HEMTs structures. The band structure across the whole heterostructure was established by solving self-consistently the Schrödinger and Poisson equations and by taking into account the experimentally determined strain state of each layer. Continuous and quasi-continuos photoluminescence reveal the presence of a broad emission band at an energy around 50 meV below the exciton emission and whose energy blueshifts with increasing excitation power density, until it is completely quenched due to the complete screening of the internal electric field. Time-resolved measurements show that the emission arising from the two-dimensional hole gas can be assigned to the recombination of holes in the potential well with electrons located in the top GaN as well as electron from the bottom AlGaN, each of them displaying different decay times due to unequal electric fields. Besides the optical demonstration of a two-dimensional hole gas in a nitride-based heterostructure, our work highlights the optical recombination processes involved in the emission from such a hole gas.

cond-mat.mtrl-sci↗

arXiv2023

Lasing in a ZnO waveguide: clear evidence of polaritonic gain obtained by monitoring the continuous exciton screening

The stimulated emission of exciton-polaritons was proposed as a means of lowering the lasing threshold because it does not require the dissociation of excitons to obtain an electron-hole plasma, as in a classical semiconductor laser based on population inversion. In this work we propose a method to prove unambiguously the polaritonic nature of lasing by combining experimental measurements with a model accounting for the permittivity change as a function of the carrier density. To do so we use angle resolved photoluminescence to observe the lasing at cryogenic temperature from a polariton mode in a zinc oxide waveguide structure, and to monitor the continuous shift of the polaritonic dispersion towards a photonic dispersion as the optical intensity of the pump is increased (up to 20 times the one at threshold). This shift is reproduced thanks to a model taking into account the reduction of the oscillator strength and the renormalization of the bandgap due to the screening of the electrostatic interaction between electrons and holes. Furthermore, the measurement of the carriers lifetime at optical intensities in the order of those at which the polariton lasing occurs enables us to estimate the carrier density, confirming it is lower than the corresponding Mott density for zinc oxide reported in the literature.

cond-mat.mtrl-sci↗

arXiv2020

Charge and spin transport over record distances in GaAs metallic n-type nanowires : I photocarrier transport in a dense Fermi sea

We have investigated charge and spin transport in n-type metallic GaAs nanowires (~ 10^17 cm^-3 doping level), grown by hydride vapor phase epitaxy (HVPE) on Si substrates. This was done by exciting the nanowire by tightly-focussed circularly-polarized light and by monitoring the intensity and circular polarization spectrum as a function of distance from the excitation spot. The spin-polarized photoelectrons give rise to a well-defined feature in the nearbandgap spectrum, distinct from the main line due to recombination of the spin-unpolarized electrons of the Fermi sea with the same minority photoholes. At a distance of 2 (micro)m, only the main line remains, implying that photoelectrons have reached a charge thermodynamic equilibrium with the Fermi sea. However, although no line is present in the intensity spectrum at the corresponding energy, the circular polarization is still observed at the same energy in the spectrum, implying that photoelectrons have preserved their spin orientation and that the two spin reservoirs remain distinct. Investigations as a function of distance to the excitation spot show that, depending on excitation power, a photoelectron spin polarization of 20% can be transported over a record distance of more than 20 (micro)m. This finding has potential applications for long distance spin transport in n-type doped nanowires.

cond-mat.mes-hall↗